With the spread of Internet in recent years, the use form of the computer has been changed, e.g., to the form of processing a great volume of moving picture data and sound data with a personal computer. Along with this trend, storage capacity required of magnetic recording media, such as hard discs, has increased.
In a hard disc apparatus, a magnetic head slightly floats from the surface of a magnetic disc with the rotation of the magnetic disc and magnetic recording is performed by non-contact recording. This mechanism prevents the magnetic disc from breaking by the contact of the magnetic head and the magnetic disc. With the increase of density of magnetic recording, the floating height of the magnetic head is gradually decreased, and now the floating height of from 10 to 20 nm has been realized by using a magnetic disc comprising a specularly polished hyper-smooth glass support having provided thereon a magnetic recording layer. In a magnetic recording medium, a CoPtCr series magnetic layer and a Cr undercoat layer are generally used, and the direction of easy magnetization of the CoPtCr series magnetic layer is controlled in the direction of in-plane of the film by the Cr undercoat layer by increasing the temperature as high as 200 to 500° C. Further, the magnetic domain in the magnetic layer is segregated by accelerating the segregation of Cr in the CoPtCr series magnetic layer. Areal recording density and recording capacity of hard disc drive have markedly increased during the past few years by technological innovation, e.g., the floating height reduction of the head, the improvement of the structure of head, and the improvement of the recording film of disc.
With the increase of throughput of digital data, there arises a need of moving a high capacity data, such as moving data, by recording on a replaceable medium. However, since the support of a hard disc is made of a hard material and the distance between head and disc is extremely narrow as described above, there is the fear of happening of accident by the collision of head and disc, and entraining dusts during operation when a hard disc is used as a replaceable medium such as a flexible disc and a rewritable optical disc, and so a hard disc cannot be used. Further, widespread in-plane recording media confront a problem, such as instability of recording signals due to thermal fluctuation in further higher density recording.
As the means for solving these problems, a magnetic recording medium provided with a magnetic layer on switching layer structure is proposed. The signal recorded on the magnetic layer can be diminished the influence of thermal fluctuation even in higher density recording, since the magnetic signal is stabilized by switched connection as compared with the signal recorded on an ordinary in-plane recording medium. As such a technique, the method disclosed, e.g., in JP-A-2001-56921 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) can be used. However, when a flexible polymer support is used as the support, not only high temperature cannot be used in magnetic layer-forming but Ru or the like used in the switching layer is great in film stress, thus the support deforms.
Further, when a high temperature sputtering film-forming method is used in manufacturing a magnetic recording medium, not only productivity is poor but the cost in mass production increases, thus the magnetic recording medium cannot be manufactured inexpensively.
On the other hand, a flexible disc comprises a flexible polymer film support and is excellent in replaceability, since it is a medium capable of contact recording, and so the disc can be produced inexpensively. However, commercially available flexible discs nowadays have such a structure that the recording layer is formed by coating a magnetic substance on a polymer film together with a polymer binder and an abrasive. Therefore, the high density recording characteristics of the magnetic layer of flexible discs are inferior to those of hard discs having a magnetic layer formed by sputtering, and the achieved recording density of flexible discs is only {fraction (1/10)} or less of that of hard discs.
Accordingly, a ferromagnetic metal thin film flexible disc having a recording layer formed by the same sputtering method as in hard discs is suggested. However, when the same magnetic layer as that of hard discs is tried to be formed on a polymer film, the polymer film is greatly damaged by heat and it is difficult to put such a flexible disc to practical use. Further, since the contact of a head with a medium is inevitable, a hard protective layer is indispensable. Therefore, it is also suggested to use highly heat resisting polyimide and aromatic polyamide films as the polymer films, but these heat resisting films are very expensive and it is also difficult to put them to practical use. When a magnetic layer is tried to be formed with cooling so as not to give thermal damage to the polymer films, the magnetic characteristics of the magnetic layer are insufficient, thus recording density can be hardly improved.
On the other hand, it has come to be known that when a ferromagnetic metal thin film comprising a ferromagnetic metal alloy and a nonmagnetic oxide is used, almost the same magnetic characteristics as those of the CoPtCr series magnetic layer formed under a condition of from 200 to 500° C. can be obtained even when a recording layer is formed under room temperature. Such a ferromagnetic metal thin film comprising a ferromagnetic metal alloy and a nonmagnetic oxide has a so-called granular structure which is proposed in hard discs, and those disclosed in JP-A-5-73880 and JP-A-7-311929 can be used. Nevertheless, the problem of thermal fluctuation remains unsolved in higher density recording.
In direct read after write (write-once read-many) type and rewritable optical discs represented by DVD-R/RW, the head and the disc are not close to each other as in a magnetic disc, therefore they are excellent in replaceability and widespread. However, from the thickness of light pickup and economical viewpoints, it is difficult for optical discs to use such a disc structure that both surfaces can be used as recording surfaces as in a magnetic disc, which is advantageous for improving capacity. Further, optical discs are low in areal recording density and data transfer speed as compared with magnetic discs, and so their performance is not sufficient yet as rewritable high capacity recording media.
As described above, rewritable replaceable high capacity recording media which satisfy performance, reliability and economic requirement are not found yet, although there is a great demand for them.